1. Field of the Invention
The present invention relates to a method of and an apparatus for recycling a used brine.
Brine is known as a solution or liquid of a low freezing point used as a low temperature medium and includes organic and inorganic brine. The brine is generally used as a coolant, an anti-freezing liquid or a heat transfer medium in either stock form or water-diluted form.
2. Description of the Prior Art
It is well known that the brine is largely used as a heat transfer medium in various applications such as, for example, an automobile engine cooling system, an ice arena or skating rink, an air-conditioning system, and a foodstuff refrigerating system. The organic brine used as an anti-freezing liquid or a coolant is prepared by mixing a base component, such as glycol or alcohol with one or more additives, for example, a corrosion inhibitor, an antioxidant, a defoaming agent, a coloring agent and so on.
In view of the presence of the additives, the brine exhibits satisfactory a corrosion inhibiting function and/or any other function dependent on the type of the additives used at an early stage subsequent to the initial use thereof. However, with passage of time, the additive such as one or more of a corrosion inhibitor, an antioxidant, a defoaming agent and others mixed in the brine depletes and deteriorates and the brine containing such additive eventually loses effective functions that ought to be exhibited by the type of the additives used.
Consequent upon degradation of the additives contained in the brine, metals comprising or located within the system in which the brine is used for an intended purpose begin to corrode and various degradation products such as metal ions and/or corrosion byproducts come to accumulate in the brine. The effective term of use of the brine that has accumulated a substantial quantity of the degradation products is generally considered to have run out and the brine is therefore replaced with a fresh brine solution. Although the effective term of use of the brine varies depending on the particular application and/or the frequency of use in the system, the effective term of use is generally considered to be 2 to 3 years.
In any event, when the used brine in the system is replaced with a substantially equal quantity of fresh brine, the used brine is recovered from the system. The used brine having been recovered from the system for the replacement with the fresh brine must be treated or otherwise disposed by a specialized waste disposal industry and, therefore, the user of the brine is burdened economically. Drainage of even a small quantity of the used brine into a drainage ditch or a river poses a critical problem leading to environmental disruption.
In view of the above, the used brine, i.e., the brine having accumulated a substantial quantity of the degradation products, can be recycled and, for this purpose, various recycling methods have hitherto been suggested and practiced. One of them is a so-called distillation method in which a distillation process is employed to recover the base component, that is, glycol or alcohol, of the used brine, and another one of them is a filtration method in which a filtering membrane having a multiplicity of micropores of 2 to 5 .mu.m in pore size is employed to remove the degradation products from the used brine.
According to the known distillation method, the base component of the used brine is recovered by distilling the used brine as described above. However, the brine relatively widely used in industries is generally diluted with water to 30 to 60% concentration and, therefore, the used brine can hardly be distilled efficiently, rendering the distillation method industrially unprofitable.
On the other hand, the known filtration method is, although effective to remove suspended particles from the used brine, ineffective to remove metal ions dissolved in the used brine. The presence of a large amount of the dissolved metal ions indicates a secondary corrosion has occurred in the metals comprising or located within the system to such an extent that an extra addition of one or more corrosion inhibitors will no longer exhibit a corrosion inhibiting function. Accordingly, the known filtration method little results in the used brine being recycled even though it has been filtered.
The inorganic brine is known as a solution of inorganic compounds such as, for example, calcium chloride and is very corrosive to metals comprising or located within the system, tending to form metal ions and corrosion byproducts relatively quickly. Because of this, in recycling the inorganic brine, the used inorganic brine is filtered through a mass of sands to remove the corrosion byproducts and other degradation products, but this technique is ineffective to remove dissolved metal ions from the used inorganic brine, posing a problem associated with the secondary corrosion as discussed above.
The heat transfer medium when used in the system similarly results in corrosion of metals comprising or located within the system, when it has degraded or deteriorated as a result of use for a period of time. Even the heat transfer medium accumulates the metal ions with passage of time of use thereof and eventually brings about the secondary corrosion. Therefore, even the used heat transfer medium is, when degraded or deteriorated, replaced with a fresh heat transfer medium.
It is most preferred to treat the used brine to yield a recycled product which may be introduced in the system for reuse, rather than disposing of as a waste product. However, as discussed hereinbefore, when the organic brine prepared by mixing the base component, i.e., glycol or alcohol, with the additives, for example, a corrosion inhibitor, is used as an anti-freezing liquid or a coolant, the base component is little deteriorated chemically, but the additives are extremely degraded with passage of time. Because of this, the organic brine used in the automobile engine cooling system comes to contain metal salts of the corrosion inhibitor used, metal oxides, precipitants such as gum, and a small quantity of metal ions and organic acids which are oxidized products of the base component of the brine, that is, glycol or alcohol, all mixed in the organic brine.
Of these impurities contained in the used organic brine, the precipitants can easily be removed by any known filtration method, but the filtration method fails to remove the metal ions. The organic acid metal salts do not precipitate because of chelation and remain dissolved in the used brine. In general, the used brine contains organic acids in a quantity ranging from some milligram per liter to some hundreds milligram per liter and metal ions in a quantity comparable to the quantity of the organic acids. As indicated above, the presence of the metal ions in the used brine, even though the used brine has been recycled, contributes to the secondary corrosion of the metals comprising or located within the system. In view of this, in order for the brine to be satisfactorily recycled, it is necessary to remove the dissolved metal ions thereby to avoid corrosion by the organic acids.
Similarly, as discussed above, the metal ions accumulate in the inorganic brine or the heat transfer medium.
Other known brine recycling methods are disclosed in, for example, U.S. Pat. Nos. 5,091,081 and 4,946,595. According to the first mention U.S. patent, a method is disclosed which generally comprises a prefiltration of the coolant, removed from an internal combustion engine cooling system, followed by a filtration of the prefiltered coolant with the use of a precision filtering membrane having a multiplicity of fine perforations. In this known method, no chemical treatment is effected and, therefore, no metal ion can be removed with the finely perforated filtering membrane.
The second mentioned U.S. patent discloses a method in which an oxidizing agent such as air or hydrogen peroxide is aerated or added into the coolant which has been removed from an internal combustion engine cooling system to reduce the unwanted dissolved metals and metal based byproducts, contained in the used coolant, into metal oxides which are subsequently removed by filtration with the use of first and second filters fluid-connected in series with each other and having respective perforations of 20 to 30 microns in size and 1 to 5 microns in size. It has, however, been found that, because the oxidizing agent such as air or hydrogen peroxide is little reactive with the metal ions present in the used coolant, the metal ions cannot be removed satisfactorily.
The Japanese Laid-open Patent Publication No. 3-43617, published in 1991, discloses an addition of a high molecular electrolytes and sodium dimethyl dithiocarbomate to the engine coolant to precipitate sulfides and metal ions, followed by a filtration of the suspended particles of 10 .mu.m or more in particle size. However, as shown in the graph of FIG. 2 showing a measured distribution of distributed particle sizes in the coolant, about 30% of the suspended particles formed by the metal ion precipitants have a particle size smaller than 10 .mu.m and, therefore, even though the suspended particles of 10 .mu.m or more are filtered out, an entire amount of the suspended particles cannot be removed.
It is to be noted that FIG. 2 illustrates a graph showing the distribution of particle sizes of the suspended particles contained in a truck engine coolant, which was obtained by mixing 400 ppm of sodium dimethyl dithiocarbomate into the coolant and allowing the resultant mixture to react for a week.
It may be contemplated to add a coagulating agent to the coolant after the suspended particulate matter has been formed to therein by the use of a high molecular electrolyte, thereby flocculate the suspended particles so as to have an increased size, so that the suspended particles of an increased size can subsequently be removed by filtration. However, the coagulating agent used tends to remain in the recycled product and, when the recycled product is reused in the engine cooling system, minute particles present in the recycled coolant are flocculated in the presence of the residual coagulating agent, leaving unwanted flocs in the recycled coolant within the engine cooling system.
In any event, none of the prior art methods is found to be effective to treat the used brine to yield a satisfactorily reusable product.